Electromagnetic energy receivers are known devices that detect RF energy, including RF energy in the microwave frequency spectrum. In one application such receivers are incorporated within conventional radar systems to receive and detect reflected radar signals. A predominant class of radar transmitters generates and emits electronic pulses; that is, RF pulses having a predetermined duration or width and other measurable electrical characteristics. In operation of the radar those RF pulses are propagated into space by the transmitting antenna. Typically these pulses are generated at a preset pulse repetition frequency, or, simply PRF. Should the emitted pulse be incident upon an electromagnetic energy reflecting object within the space, such as the metal surface of an aircraft moving through the space, a portion of the incident pulse is reflected back to and is received by the radar system's receiving antenna. The antenna couples the received RF pulse to the receiver, which processes the pulse and, in conjunction with the radar system's display apparatus, displays information from which intelligence concerning the movement and position of that object is communicated to persons operating the detection system.
"Noise" exists in all such electronic systems. The noise, more appropriately electronic noise, is a limiting factor in the electronic systems ability to detect and process received RF signals. If the energy level of the electronic noise in the system gets higher and higher with respect to the energy level of the received RF pulses, a point is reached at which it is no longer possible to recover the received RF pulse from the noise. The detection sensitivity of RF receiver is thus determined by the noise within the receiver. It is the suppression of a dominant portion of that kind of noise which the present invention addresses.
A typical radar receiver includes a "front end" that receives incoming RF signals picked up at the antenna and couples them to an RF mixer. The RF mixer combines the RF input signal with another signal supplied by a local oscillator within the receiver to generate an intermediate frequency or "IF" signal. The IF signal is processed; that is, amplified, filtered, and like conventional treatment. Following a stage of video detection the signal is again processed to finally be applied to the display apparatus, whether of an audio or visual type.
Radar systems may be either noncoherent or coherent. The latter kind can offer better signal to noise ratio and, hence, is often preferred, despite the increased complexity and cost of the circuitry. It is well known to those familiar with the art that in a coherent system, integration or summing of N pulses gives an N-fold improvement in the signal-to-noise, S/N, ratio, whereas in non-coherent system only an improvement of .sqroot.N can be obtained.
The radar receiver's IF signal contains basically two internal noise components; the semi-conductor noise; that is, the "shot" noise from the RF mixer and/or the field effect transistor, typically used in modern front ends; and, secondly, the noise produced by the receiver's local oscillator. The noise signals are internal to the system. Of the two types, the noise contributed by the local oscillator is invariably dominant.
Until recently the receiving system sensitivity was limited by so called "radar clutter" which is the various RF signals reflected from objects within the radar's field of view that are not of interest, such as ground clutter (reflections from vegetation, buildings, etc.) for an air-to-air radar, while the interest is reversed for a terrain following radar. The use of more selectives highly directional transmitting and/or receiving antennas, with lower side lobes, and the use of more sophisticated signal processing techniques, such as the use of improved signal processing algorithms in digital equipment associated with present day radar receivers, eliminates much of the clutter. The signal to clutter ratio can thus be pushed below the receivers signal to noise ratio in a number of modern radar systems. Hence, the sensitivity of the receiver is then limited by the level of electronic noise.
Noise suppression in RF receivers improves the receiving systems ability to detect and process reflected RF energy. Moreover noise suppression at the receiver is useful even if the existing noise level specifications are retained. Thus, for example, the transmitter noise specification can be significantly relaxed if noise suppression at the receiving end of the system suppresses the transmitted electronic noise. With a lesser noise specification for the transmitter, favorable design changes may be made to the transmitter portion of the system that incorporates additional or different features or technologies that otherwise could not presently be employed because they produce too much noise. For example, an injection or phase locked IMPATT transmitter, a relatively high powered RF device for its size, can be replaced with a free running IMPATT transmitter, which is a rather "noisy" device. This results in a lower cost transmitter having greater performance features in respects other than noise, particularly those having improved frequency agility characteristics.
A principle object of the invention is to eliminate phase noise in received RF pulsed signals. An ancillary object of the invention is to reduce the phase noise limitations of transmitters by suppressing any phase noise introduced into the transmitting signal at the receiver so as to provide a more versatile and/or economic transmitter in a transmitter receiver system combination, such as used in a radar system.
In attaining the foregoing objects, applicant's have discovered a novel pulse delay circuit. In that circuit means are provided to generate copies of the RF pulsed signal to form a train of contiguous pulses, thereby effectively retaining a copy of the RF input pulse over a prescribed time interval. The pulse delay circuit has application as an element of the principal invention and in other applications as well, including, but not limited to, electronic decoys in Electronic Warfare applications. Accordingly it is an ancillary object of the invention to provide an pulse delay circuit of novel and inexpensive structure. A still further object is to provide a pulse delay circuit that permits manufacture of low cost electronic decoys.